Low alloyed steel powder

ABSTRACT

A water atomized pre-alloyed iron-based steel powder which includes by weight-%: 0.4-2.0 Cr, 0.1-0.8 Mn, less than 0.1 V, less than 0.1 Mo, less than 0.1 Ni, less than 0.2 Cu, less than 0.1 C, less than 0.25 O, less than 0.5 of unavoidable impurities, and the balance being iron.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.12/810,224, filed on Aug. 31, 2010, which is a U.S. national stage ofInternational Application No. PCT/SE2008/051511, filed on Dec. 18, 2008,which claims the benefit of U.S. Provisional Application No. 61/017,038,filed on Dec. 27, 2007, and claims the benefit of Swedish ApplicationNo. 0702892-1, filed on Dec. 27, 2007. The entire contents of each ofU.S. U.S. application Ser. No. 12/810,224, International Application No.PCT/SE2008/051511, U.S. Provisional Application No. 61/017,038, andSwedish Application No. 0702892-1 are hereby incorporated herein byreference in their entirety.

FIELD OF THE INVENTION

The present invention concerns a low alloyed iron-based powder, a powdercomposition containing the powder and other additives, and a componentmade by compaction and sintering of the iron-base powder compositioncontaining the new low alloyed steel powder. The mechanical propertiesof the component made from the invented powder are comparable with themechanical properties of a component made from a more highly alloyed,and more expensive diffusion bonded powder.

BACKGROUND OF THE INVENTION

In industries the use of metal products manufacturing by compaction andsintering metal powder compositions is becoming increasingly widespread.A number of different products of varying shape and thickness are beingproduced and the quality requirements are continuously raised at thesame time as it is desired to reduce the cost. As net shape components,or near net shape components requiring a minimum of machining in orderto reach finished shape, are obtained by pressing and sintering of ironpowder compositions in combination with a high degree of materialutilization, this technique has a great advantage over conventionaltechniques for forming metal parts such as molding or machining from barstock or forgings.

One problem connected to the press and sintering method is however thatthe sintered component contains a certain amount of pores decreasing thestrength of the component. Basically there are two ways to overcome thenegative effect on mechanical properties caused by the componentporosity. 1) The strength of the sintered component may by increased byintroducing alloying elements such as carbon, copper, nickel molybdenumetc. 2) The porosity of the sintered component may be reduced byincreasing the compressibility of the powder composition, and/orincreasing the compaction pressure for a higher green density, orincreasing the shrinkage of the component during sintering. In practicea combination of strengthening the component by addition of alloyingelements and minimizing the porosity are applied. Thus, variouscompositions of low-alloyed steel powders, and methods for compaction ofthese powders are known for production of PM component showing highstrength and hardness. However, a characteristic property of PMcomponents is a relative low toughness compared to wrought steelmaterials. The so called diffusion alloyed iron based powders, having arelatively high compressibility despite being “highly” alloyed, providespossibilities for producing compacted and sintered bodies having a hightoughness and high elongation in combination with a high strengthcompared to pre-alloyed powders.

However, a drawback related to presently used diffusion alloyed powdersis their relatively high content of costly alloying elements such asmolybdenum and nickel. It has now unexpectedly been found that by acareful selection of a combination of the alloying elements chromium andmanganese, at relatively low content, a pre-alloyed powder is obtainedgiving a pressed and sintered body mechanical properties with respect toelongation and strength at the same level or close to the values whichcan be obtained by using a more alloyed diffusion bonded powder.

U.S. Pat. No. 4,266,974 discloses examples of alloyed powders outsidethe claimed scope containing only manganese and chromium asintentionally added alloying elements. The examples contains 2.92% ofchromium in combination with 0.24% of manganese, 4.79% of chromium incombination with 0.21% by weight of manganese or 0.55% of chromium incombination with 0.89% by weight of manganese.

In Japanese patent publication number J59173201 a method of reductionannealing of a low alloyed steel powder containing chromium, manganeseand molybdenum, one example shows a powder having a chromium content of1.14% by weight and a manganese content of 1.44% by weight as the onlyintentionally added alloying elements.

A chromium, manganese and molybdenum based pre-alloyed steel powder isdescribed in U.S. Pat. No. 6,348,080. WO03/106079 teaches a chromium,manganese and molybdenum alloyed steel powder having lower content ofalloying elements compared the steel powder described in U.S. Pat. No.6,348,080. The powder is suitable to form bainitic structures at acarbon content above about 0.4% by weight.

OBJECTS OF THE INVENTION

An object of the invention is to provide an alloyed iron-based powdersuitable for producing compacted and sintered components, the powderbeing essentially free from costly alloying elements such as molybdenumand nickel.

Another object of the invention is to provide a powder capable offorming compacted and sintered components having good elongation,tensile strength and yield strength.

Another object of the invention is to provide a sintered part having theabove mentioned properties.

SUMMARY OF THE INVENTION

At least one of these objects is accomplished by:

-   -   A water atomized pre-alloyed iron-based steel powder which        comprises by weight-%: 0.4-2.0 Cr, 0.1-0.8 Mn, less than 0.1 V,        less than 0.1 Mo, less than 0.1 Ni, less than 0.2 Cu, less than        0.1 C, less than 0.25 O, less than 0.5 of unavoidable        impurities, and the balance being iron.    -   An iron-based powder composition based on the steel powder and        mixed with 0.35-1% by weight of the composition of graphite,        0.05-2% by weight of the composition of lubricants and        optionally copper in an amount up to 3%, hard phase materials        and machinability enhancing agents.    -   A method of producing a sintered component comprising the steps        of;    -   a) preparing the iron-based steel powder composition based on        the steel powder,    -   b) subjecting the composition to compaction between 400 and 2000        MPa,    -   c) sintering the obtained green component in a reducing        atmosphere at temperature between 1000-1400° C.,    -   d) optionally forging the heated component at a temperature        above 500° C. or subjecting the obtained sintered component to a        heat treatment or hardening step.    -   A sintered component produced by the method having a        pearlitic/ferritic microstructure

The steel powder has low and defined contents of chromium and manganeseand is essentially free from molybdenum, nickel and vanadium.

DETAILED DESCRIPTION OF THE INVENTION Preparation of the Iron-BasedAlloyed Steel Powder

The steel powder is produced by water atomization of a steel meltcontaining defined amounts of alloying elements. The atomized powder isfurther subjected to a reduction annealing process such as described inthe U.S. Pat. No. 6,027,544, herewith incorporated by reference. Theparticle size of the steel powder could be any size as long as it iscompatible with the press and sintering or powder forging processes.Examples of suitable particle size is the particle size of the knownpowder ABC100.30 available from Höganäs AB, Sweden, having about 10% byweight above 150 μm and about 20% by weight below 45 μm.

Contents of the Steel Powder

Chromium serves to strengthen the matrix by solid solution hardening.Furthermore, chromium will increase the hardenability, oxidationresistance and abrasion resistance of the sintered body. A content ofchromium above 2.0 wt % will however reduce the compressibility of thesteel powder and render the formation of a ferritic/pearliticmicrostructure more difficult. Preferably from the viewpoint ofcompressibility the maximum content is about 1.8 wt %, even morepreferred 1.5 wt %. A Cr content below 0.4% by weight will haveinsignificant effect on desired properties. Preferably, the chromiumcontent is at least 0.5 wt %.

Manganese will, as for chromium, increase the strength, hardness andhardenability of the steel powder. A content above 0.8 wt % willincrease the formation of manganese containing inclusion in the steelpowder and will also have a negative effect on the compressibility dueto solid solution hardening and increased ferrite hardness. Preferably,the manganese content is below 0.7 wt %, even more preferably themanganese content is below 0.6 wt %. If the manganese content is below0.1% desired properties cannot be obtained, furthermore, it will notpossible to use recycled scrap unless a specific treatment for thereduction during the course of the steel manufacturing is carried out,For these reasons the manganese content is preferably at least 0.2 wt %,even more preferred 0.3 wt %. Thus the manganese content should bebetween 0.1-0.8 wt %, preferably 0.2-0.7 wt %, even more preferred0.3-0.6 wt %.

It has also been found that in order to obtain a sufficiently highcompressibility the total amount of chromium and manganese, which tosome extent are exchangeable with each other, should not be more than2.5% by weight, preferably not more than 2.3% by weight, most preferablynot more than 2.0% by weight.

In one embodiment with low chromium contents in the range of 0.4-0.6 wt% Cr, the low chromium content is compensated by a comparably highmanganese content in the range of 0.6-0.8 wt %, preferably 0.7-0.8 wt %.This is embodiment is advantageous since manganese is less expensivethan chromium.

In another embodiment, when the chromium content is at least 0.7 wt %the manganese content is at most 0.5 wt %, and when the chromium contentis at least 1.0 wt % the manganese content is at most 0.4 wt %,preferably at most 0.3 wt %. By having high chromium content, themanganese content may be kept lower, thereby minimizing any formation ofmanganese containing inclusion in the steel powder.

Oxygen suitably is at most 0.25 wt %, to prevent formation of oxideswith chromium and manganese that impairs strength and compressibility ofthe powder. For these reasons oxygen preferably is at most 0.18 wt %.

Vanadium and nickel should be less than 0.1 wt % and copper less than0.2 wt %. A too high content of these elements will have a negativeeffect on compressibility and may increase costs. Also, the presence ofnickel will suppress ferrite formation, thus promoting a brittlepearlitic/bainitic structure. Molybdenum should be less than 0.1 wt % toprevent bainite to be formed as well as to keep costs low sincemolybdenum is a very expensive alloying element.

Carbon in the steel powder shall be at most 0.1% by weight and oxygen atmost 0.25% by weight. Higher contents will unacceptably reduce thecompressibility of the powder. For the same reason nitrogen shall bekept less than 0.1 wt %.

The total amount of inevitable impurities should be less than 0.5% byweight in order not to impair the compressibility of the steel powder oract as formers of detrimental inclusions.

Iron-Based Powder Composition

Before compaction the iron-based steel powder is mixed with graphite andlubricants. Graphite is added in an amount between 0.35-1.0% by weightof the composition and lubricants are added in an amount between0.05-2.0% by weight of the composition. In a certain embodiment copperin the form of copper powder may be added in an amount up to 3% byweight. In another embodiment nickel powder up to 5% by weight with orwithout additional copper powder may be added to the composition byadmixing.

Amount of Graphite

In order to enhance strength and hardness of the sintered componentcarbon is introduced in the matrix. Carbon is added as graphite inamount between 0.35-1.0% by weight of the composition. An amount lessthan 0.35% by weight will result in a too low strength and an amountabove 1.0% will result in an excessive formation of carbides yielding atoo high hardness, insufficient elongation and impair the machinabilityproperties. In case that after sintering or forging, the component isheat treated with a heat treatment process including carburizing; theamount of added graphite may be less than 0.35%.

Amount of Copper

Copper is a commonly used alloying element in the powder metallurgicaltechnique. Copper will enhance the strength and hardness through solidsolution hardening. Copper will also facilitate the formation ofsintering necks during sintering as copper melts before the sinteringtemperature is reached providing so called liquid phase sintering whichis much faster than sintering in solid state. In a certain embodimentcopper may be added in an amount up to 3% by weight.

Amount of Nickel

Nickel is a commonly used alloying element in the powder metallurgicaltechnique. Nickel will enhance the strength and hardness through solidsolution hardening. Nickel will also strengthen the sintering necksduring sintering. In a certain embodiment nickel may be added in anamount up to 5% by weight.

Amount of Lubricants

Lubricants are added to the composition in order to facilitate thecompaction and ejection of the compacted component. The addition of lessthan 0.05% by weight of the composition of lubricants will haveinsignificant effect and the addition of above 2% by weight of thecomposition will result in a too low density of the compacted body.Lubricants may be chosen from the group of metal stearates, waxes, fattyacids and derivates thereof, oligomers, polymers and other organicsubstances having lubricating effect.

Other Substances

Other substances such as hard phase materials and machinabilityenhancing agents, such as MnS, MoS₂, CaF₂, and different kinds ofminerals etc. may be added.

Sintering

The iron-based powder composition is transferred into a mold andsubjected to a compaction pressure of about 400-2000 MPa to a greendensity of above about 6.75 g/cm³. The obtained green component isfurther subjected to sintering in a reducing atmosphere at a temperatureof about 1000-1400° C., preferably between about 1100-1300° C.

Post Sintering Treatments

The sintered component may be subjected to a hardening process forobtaining desired microstructure through heat treatment includingcooling at a controlled cooling rate. The hardening process may includeknown processes such as case hardening, nitriding, induction hardeningand the like. In case that heat treatment includes carburizing theamount of added graphite may be less than 0.35%.

Alternatively, the sintered component may be subjected to a forgingoperation in order to reach full density. The forging operation may beperformed either directly after the sintering operation when thetemperature of the component is about 500-1400° C., or after cooling ofthe sintered component, the cooled component is then reheated to atemperature of about 500-1400° C. prior to the forging operation.

Other types of post sintering treatments may utilized such as surfacerolling or shot peening which introduces compressive residual stressesenhancing the fatigue life.

Properties of the Finished Component

The present invention provides a new iron-based pre-alloyed powder forthe manufacture of sintered components having tensile strength andelongation comparable with the corresponding values obtained from adiffusion bonded powder containing higher total amount of alloyingelements, and more expensive alloying elements such as nickel andmolybdenum. Especially, the present invention provides a chromium andmanganese pre-alloyed iron-based powder, a composition containing thepowder, as well as a compacted and sintered component made from thepowder composition. The compacted and sintered component exhibits avalue for elongation above 2% in combination with a yield strength ofabout 500 MPa. The microstructure is pearlitic or pearlitic/ferritic.

EXAMPLES

Various pre-alloyed iron-based steel powders 1-5 were produced by wateratomizing of steel melts. The obtained raw powders were further annealedin a hydrogen atmosphere at 1160° C. followed by gently grinding todisintegrate the sintered powder cake. The particle size of the powderswas below 150 μm. Table 1 shows the chemical compositions of thedifferent powders. Powder 6 was DISTALOY AB, a commercialdiffusion-alloyed powder available from Höganäs, Sweden, and based onthe high-purity atomized powder ASC100.29 (plain iron).

TABLE 1 Cr Mn V Cu Mo Powder [%] [%] [%] [%] [%] Ni [%] C [%] O [%] 10.80 0.35 — — — — 0.003 0.090 2 0.68 0.68 — — — — 0.003 0.093 3 1.780.10 — — — — 0.009 0.062 4 (Ref) 0.92 0.03 0.11 — — — 0.005 0.043 5(Ref) 0.25 0.06 — — — — 0.003 0.039 6 — — — 1.50 0.50 1.75 0.002 0.092

Table 1 shows the chemical composition of steel powder according to theinvention and reference materials.

The obtained steel powders 1-5 were mixed with 0.5% and 0.7% by weightof the composition, respectively, of graphite UF4, available fromKropfmühle, Germany and 0.8% of Amide wax PM, available from Höganäs AB,Sweden.

Powder 4 was outside the boundaries of the present invention beingalloyed with 0.11 wt % vanadium and having a manganese content of 0.03wt %. Powder 5 had both manganese content and chromium content below theboundaries of the present invention.

A reference mix, based on DISTALOY AB (powder 6) was also prepared. Inthis case the composition prepared contained 0.5% of graphite and 0.8%of Amide Wax PM.

The obtained powder compositions were transferred to a die and compactedto form tensile tests bars at a compaction pressure of 600 MPa. Thecompacted tests bars were further sintered in a laboratory belt furnaceat 1120° C. for 30 minutes in an atmosphere of 90% nitrogen and 10% ofhydrogen.

The sintered samples were tested with respect to tensile strength andelongation according to ASTME9-89C and hardness, HV10 according to ENISO 6507-1. The samples were also analyzed with respect to the carbonand oxygen content.

Impact energy was tested in accordance with EN10045-1.

The following Table 2 shows added amount of graphite, results fromchemical analysis, and results from tensile and hardness testing.

TABLE 2 Powder Composition Added Yield Tensile based on Graphitestrength strength Elongation Hardness, powder [%] C [%] O [%] [MPa][MPa] [%] HV10 IE [J] 1 0.5 0.50 0.05 345 512 4.6 139 25 1 0.7 0.69 0.05417 627 3.0 178 23 2 0.5 0.55 0.07 371 540 3.3 168 24 2 0.7 0.72 0.07398 611 3.0 181 21 3 0.5 0.56 0.03 427 625 3.1 172 28 3 0.7 0.72 0.03496 697 2.3 195 23 4 (Ref) 0.5 0.51 0.03 379 517 2.8 156 19 4 (Ref) 0.70.70 0.03 462 610 1.6 192 15 5 (Ref) 0.5 0.48 0.02 287 391 4.2 113 23 5(Ref) 0.7 0.67 0.02 331 478 3.2 143 19 6-Dist AB 0.5 0.48 0.01 363 6102.8 178 28

Table 2 shows the amount of added graphite to the compositions, analyzedC and O content of the produced samples, as well as results from tensiletest and hardness testing of the produced samples.

At 0.7% added graphite, samples based on powder 1, 2 showed comparableor better values than DISTALOY AB mixed with 0.5% graphite powder foryield strength, tensile strength, elongation, and hardness. Impactenergy was slightly below but still sufficiently good, slightly betterfor powder 1 than for powder 2.

Already at 0.5% added graphite, samples based on powder 3 showedcomparable or better values than DISTALOY AB mixed with 0.5% graphitepowder for yield strength, tensile strength, elongation. Also impactenergy and hardness matches DISTALOY AB.

For samples based on powder 4 elongation and impact energy are muchlower than the values of DISTALOY AB at comparable tensile strength. Forsamples based on powder 5 it can be seen that impact energy andelongation are decreasing with increasing carbon content and would bemuch lower if even higher graphite additions would be applied toincrease tensile strength to a comparable level with DISTALOY AB.

1. A water atomized pre-alloyed iron-based steel powder which comprisesby weight-%: 0.4-2.0 Cr, 0.1-0.8 Mn, less than 0.1 V, less than 0.1 Mo,less than 0.1 Ni, less than 0.2 Cu, less than 0.1 C, less than 0.25 O,less than 0.5 of unavoidable impurities, and the balance being iron. 2.A powder according to claim 1, wherein the content of Cr is at most 1.8weight-%, preferably at most 1.5 weight-%.
 3. A powder according toclaim 1, wherein the content of Cr is at least 0.5 weight-%.
 4. A powderaccording to claim 1, wherein the content of Mn is at least 0.2weight-%, preferably at least 0.3 weight-%.
 5. A powder according toclaim 1, wherein the content of Mn is at most 0.7 weight-%, preferablyat most 0.6 weight-%.
 6. A powder according to claim 1, wherein the sumof the chromium and manganese content is less than 2.5 wt %, preferablyless than 2.3 wt % and most preferably less than 1.9% by weight.
 7. Apowder according to claim 1, wherein the content of Cr is 0.4-0.6weight-%, and the content of Mn is 0.6-0.8 weight-%, preferably thecontent of Mn is 0.7-0.8 weight-%.
 8. A powder according to claim 1,wherein the content of Cr is at least 0.7 weight-%, and the content ofMn is at most 0.5 weight-%.
 9. A powder according to claim 1, whereinthe content of Cr is at least 1.0 weight-%, and the content of Mn is atmost 0.4 weight-%, preferably at most 0.3 weight-%.
 10. An iron-basedpowder composition comprising a steel powder according to claim 1 mixedwith 0.35-1% by weight of the composition of graphite, 0.05-2% by weightof the composition of lubricants and optionally copper in an amount upto 3%, and optionally hard phase materials and machinability enhancingagents.
 11. A method of producing a sintered component comprising thesteps of: a) preparing an iron-based steel powder mixture having thecomposition claimed in claim 10, b) subjecting the composition tocompaction between 400 and 2000 MPa, c) sintering the obtained greencomponent in a reducing atmosphere at temperature between 1000-1400° C.,d) optionally forging the heated component at a temperature above 500°C. or subjecting the obtained sintered component to a heat treatment orhardening step.
 12. A sintered component produced from the powdercomposition according to claim 10 having a pearlitic/ferriticmicrostructure.